In the fabrication of semiconductor devices, metal contacts and vias are frequently formed in contact holes and via openings on silicon wafers that have been pre-processed with insulating layers on top. Devices are then fabricated by connecting the components with metal contacts and vias to form the integrated circuit. In particular, aluminum, aluminum alloys, tungsten and tungsten alloys are frequently used for depositing into contact holes and via openings on silicon substrates. The deposition process can be carried out either in a physical vapor deposition chamber or in a chemical vapor deposition chamber.
As the dimensions of semiconductor devices continuously to shrink in the miniaturization of modern semiconductor devices to the sub-half-micron range, via openings and contact holes must also shrink. Consequently, the openings and holes to be filled have larger aspect ratios, i.e., the ratios between the depth of the opening or hole and the diameter.
Difficulties have been encountered in depositing conductive metals into via openings and contact holes that have high aspect ratios by the conventional sputtering process. As the openings or holes become smaller and deeper, the bottom and sides of an opening or hole receive fewer deposited metal particles than the top surface of the device. The end result of such a phenomenon, sometimes called a shadowing effect, is that metal layers formed by the particles hang over the opening forming an overhang. The overhang closes before the opening is completely filled as the deposition process progresses and thus creating a void in the opening or hole.
One technique used to remedy the shadowing effect of the sputtering process is to use a tungsten chemical vapor deposition (W CVD) technique for filling openings and holes that have large aspect ratios. The W CVD process solves the difficult problems in metalization to ensure enough metal continuity in contact windows and vias. The step coverage of deep openings or holes by the W CVD particles is greatly improved over that possible by any other deposition techniques. In a W CVD process conducted on a silicon substrate for filling a contact hole, the basic chemistry is represented by: EQU WF.sub.6 +3H.sub.2.fwdarw.W+6HF (Equation A) EQU 2WF.sub.6 +3Si.fwdarw.2W+3SiF.sub.4 (Equation B)
There may also be reactions between WF.sub.6 and SiH.sub.4 and furthermore, WF.sub.6 may be reduced by Al and Ti through different chemical processes.
During a W CVD deposition process, a wafer is usually held on a vacuum chuck that is heated to a temperature between about 400.degree. C. and about 500.degree. C. A shower head is positioned opposite to the wafer where WF.sub.6 , H.sub.2 or SiH.sub.4 gases are injected. Normally, a two-or three-step process is involved where SiH.sub.4 is first introduced without any flow of WF.sub.6 to initiate a deposition of a very thin seed layer of amorphous silicon as a prenucleation layer. The prenucleation process is then followed by a SiH.sub.4 +WF.sub.6 silane reduction nucleation process for depositing a thin W nucleation layer, and then the faster-rate H.sub.2 +WF.sub.6 hydrogen reduction process for bulk W deposition. During the nucleation stage, less than 100 nm of tungsten is deposited, while the bulk of the tungsten deposition is by the hydrogen reduction process. The multi-stage deposition process is designed such that during the initial nucleation stage, the silicon from the source/drain area is not consumed in the reaction since WF.sub.6 would react readily with Si. When WF.sub.6 reacts with Si from the source/drain region, a defect known as junction leakage may occur. The introduction of SiH.sub.4 first into the reaction avoids the consumption of Si from the substrate. The initial introduction of SiH.sub.4 into the reaction without WF.sub.6 for the deposition of the prenucleation layer of Si is known as a silane soak step.
In the W CVD process, a W CVD is frequently blanket-deposited onto a wafer surface and into the contact holes after a metal nucleation layer is first deposited on the entire wafer. The W deposited on the insulating layer, i.e., a SiO.sub.2 layer, is then etched off in an etchback process by a process of reactive ion etching. After the etchback process, only the thicker W in the contact holes are left. Since the process relies on the removal of all W CVD except in the contact holes, the uniformity of the W deposition and the RIE etchback process is critical for the successful formation of W contact plugs. When the process is not accurately controlled, such as with the pre-deposition of a nucleation layer of Ti/TiN, the W contact plugs may be substantially recessed after the etchback process and thus results in poor step coverage on the device.
In the deposition of contact plugs by the W CVD process, another process difficulty has been observed which is caused by the very nature of the excellent coverage capability of tungsten particles. In a CVD chamber where a wafer is held on a vacuum chuck, on the bottom side of the wafer and specifically around the edges of the wafer, tungsten particles may also deposit which causes a problem known as the backside pressure fault or BSPF. When a backside pressure fault occurs, the servo pressure (which indicates the chamber pressure) exceeds a maximum allowable value such that the deposition process must be stopped. When the backside pressure fault problem is corrected and the deposition process restarted in the process chamber, it has been noticed that the W contact plugs formed on the wafer surface, particularly around an edge portion of the wafer, are frequently defective and are lost after a W etchback process. The BSPF problem is believed to have been caused by the formation of uneven layers of WSi.sub.x from the prenucleation layer of Si, particularly at around the edges of the wafer due to a warped wafer or poorly positioned wafer on the vacuum chuck. It is believed that, when non-uniform WSi.sub.x layers are formed in the contact hole prior to the filling of the hole with bulk W, a dry etchant such as SF.sub.3 etches the WSi.sub.x film at a much faster rate than the W layer (on top of the device) during an etchback process. This results in a complete loss of the W contact plug that was formed in the contact hole. A typical defect of a lost W contact plug is shown in FIG. 1 which was taken after an etchback process has been performed. In order to eliminate the backside pressure fault problem observed in a W CVD process, the formation of uneven WSi.sub.x films and the subsequent etching of the films by a dry etchant must be avoided.
It is therefore an object of the present invention to provide a method for forming a tungsten contact plug in a tungsten CVD process without the drawbacks or shortcomings of the conventional tungsten deposition methods.
It is another object of the present invention to provide a method for forming a tungsten contact plug in a W CVD process without the plug loss defect after a backside pressure fault has occurred in the process chamber.
It is a further object of the present invention to provide a method for forming a tungsten contact plug in a W CVD process without the plug loss defect by incorporating a heat treating step on the substrate prior to the deposition of tungsten for filling the contact hole.
It is another further object of the present invention to provide a method for forming a tungsten contact plug in a W CVD process without the plug loss defect by heat treating the substrate at a temperature of at least 600.degree. C. for at least 10 seconds prior to the tungsten deposition process for filling the contact hole.
It is still another object of the present invention to provide a method for forming a tungsten contact plug in a W CVD process without the plug loss defect by heat treating the substrate at a temperature of at least 600.degree. C. and by avoiding the formation of W Si.sub.x films in the contact hole.
It is still another further object of the present invention to provide a method for forming a tungsten contact plug in a W CVD process without the plug loss defect by first depositing a silicon prenucleation layer in the contact hole and then heat treating the substrate at a temperature of at least 600.degree. C. such that substantially no WSi.sub.x is formed in the contact hole.
It is yet another further object of the present invention to provide a method for forming a tungsten contact plug in a W CVD process without the plug loss defect by first depositing a glue layer of Ti/TiN, and then depositing a silicon prenucleation layer in the contact hole prior to a heat treating process of the substrate at a temperature of at least 600.degree. C. for at least 10 seconds such that substantially no WSi.sub.x film is formed in the contact hole.